M0nica
School reports are always very formulaic and meaningless. Our Ds and DDiL will tell us bits and pieces from Parents Evenings and several problems that may be genetic in origin have also been discussed between us.
I find the intrusive part some grandparents play in family life these days quite weird. When I got married, my parents immediately respected our autonomy and the exclusiveness and allowed us to stand on our own feet.
I extended the same courtesy to my DS and DDiL. and what they invite us to be part of, or tell us we treat with confidence, we would only ever look at something, we were explicitly invited to.
I agree with M0nica, school reports are really only intended for the child and their parents. Whether they choose to share them with extended family is entirely up to them, and at 13 I think your GC is old enough to expect some privacy.
Our AC and GC live a long way from us so we have never expected to be a regular part of their lives. They are their own independent family units, we are just extended family now. We all get along well and when we're together we enjoy those times, but we don't expect constant involvement and updates on everything they do.
One of my DILs will sometimes mention their reports or even email a copy to me. I always ask if it's OK with the child that I see it, and if they said no then I would delete the email. I'll be honest and say I find the way reports are 'written' very impersonal now, and the grading is like unravelling a scientific formula!
So yes Grandma70s I do think YABU and definitely over-reacting by being so upset by it.